1. Field of the Invention
The subject invention relates to methods for testing dynamic properties of a vehicle, particularly a propensity of the vehicle to roll over during operation.
2. Description of the Prior Art
Rollover accidents have been one of the greatest vehicle safety concerns for decades, according to the National Highway Traffic Safety Administration (NHTSA). In fact, rollover accidents are the largest cause of fatalities in passenger car and light truck accidents. Rollover accidents have also been the subject of intense litigation over recent years, giving rise to a need for better alternatives to traditional testing of the propensity of vehicles to rollover.
A common standard used by the NHTSA for the propensity of vehicles to rollover is Static Stability Factor, or SSF. As the name implies, the SSF is a static measurement of a vehicle. The SSF is based on one half of the average front and rear track-width divided by the total vehicle center of gravity and height. The SSF is useful as one of many factors in determining the propensity of vehicles to rollover, but alone is insufficient. The SSF assumes that vehicles act like rigid boxes not taking into account the compliance of wheels and suspensions.
Currently, there are several vehicle testing apparatuses that purport to dynamically measure the propensity of vehicles to rollover in a controlled environment. Although such apparatuses may provide useful results for particular properties of vehicles, the apparatuses cannot accurately measure the propensity of vehicles to rollover. One reason for this is that apparatuses of the prior art have limited capability and cannot exert compound dynamic forces on vehicles akin to an actual vehicle rollover situation. Another reason for the lack of accuracy of the prior art vehicle testing apparatuses is that the methods used for performing the tests do not attempt to pinpoint a threshold force that causes wheel lift-off. For example, a centrifuge device can be used to produce lateral accelerations. The operation requires that a certain speed to be reached and then the vehicle is released to roll. Hence, any event that is simulated is only what happens after the roll is initiated. Also since a centrifuge device is used, any developed lateral accelerations are not perfectly perpendicular to the vehicle longitudinal axis and varies by the vehicle's length. Another example is a flat track road simulator which can produce roll, pitch and vertical motions of the subject vehicle. However, road simulators lack the lateral acceleration which can be an important factor in a rollover accident. In another example, a vehicle sled allows vehicles to be propelled laterally along a horizontal axis. The vehicle sled is propelled and abruptly stopped to trigger a rollover of the vehicle. The pressure used to propel the sled is not controlled accurately to match the lateral acceleration to any particular rollover maneuver, but rather is aimed to roll the vehicle over following a trip. The vehicle sled cannot exert compound dynamic forces on vehicles akin to an actual rollover situation and thus does not account for many factors that have an effect on the propensity of vehicles to rollover.
Although the propensity of vehicles to rollover can be tested through real-world driving maneuvering on test tracks, such tests have proven to be unrepeatable and unpredictable and therefore cannot be standardized, unless prohibitively expensive methods are used which would be applicable to only a limited number of rollover maneuvers. In addition, a great deal of real-world vehicle rollover situations are tripped by an obstacle, which can either be an object in a roadway or a particular structure of the roadway, such as curbs, potholes, etc. As the vehicle is turning or sliding sideways on the roadway, a side of the wheel encounters the obstacle. The side of the wheel catches on the obstacle, thus creating a fulcrum at the wheel. Vehicle rollover occurs when the moment of lateral forces around a fulcrum overcomes the moment created by the weight of the vehicle about the same fulcrum point. It is almost impossible to formulate a maneuver that will implement a tripped vehicle rollover situation in a repeatable manner on the test track due to uncontrollability and unobservability of several parameters.
Another issue with vehicle rollover testing is that each vehicle, even if of the same model, is slightly different. Such slight differences, no matter how small, can have an effect on the propensity of the vehicle to rollover. Current methods do not take this into account and generally do not test static and dynamic properties of each vehicle before performing the vehicle rollover testing. Moreover, all necessary tests cannot be performed on one testing apparatus. The resulting testing is skewed because of the slight differences, which also affect repeatability of the tests.
Thus, there remains an opportunity for a vehicle testing method for measuring the propensity of vehicles to rollover that produces repeatable results and that measures a point of wheel lift-off similar to real-world forces exerted on vehicles during tripped and untripped rollover situations without damaging the vehicle. Furthermore, there remains an opportunity to test the static and dynamic properties of each vehicle prior to testing the propensity of the vehicle to rollover for enhancing repeatability of the results.